Backport MiG commit 73fbf2504ae606dda81372701a67dad6fee0865e:

  From: Flavio Cruz <flaviocruz@gmail.com>
  Subject: Generate cpu.h with -ffreestanding

--- a/Makefile.in	2023-03-26 23:02:51.384318331 +0200
+++ b/Makefile.in	2023-03-26 23:03:03.300329369 +0200
@@ -1051,7 +1051,7 @@ uninstall-am: uninstall-binSCRIPTS unins
 .sym.symc:
 	$(AWK_V) $(AWK) -f $(srcdir)/gensym.awk $< > $@
 .symc.symo:
-	$(AM_V_CC) $(TARGET_CC) -S $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) -x c -o $@ $<
+	$(AM_V_CC) $(TARGET_CC) -S $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) -ffreestanding -x c -o $@ $<
 .symo.h:
 	$(AM_V_GEN) sed -e '/^[^*].*$$/d' -e 's/^[*]/#define/' -e 's/mAgIc[^-0-9]*//' \
 	    $< > $@
